Cytokines are proteins which are produced during the effector phases of natural and specific immunity and serve to mediate and regulate immune and inflammatory responses. Cytokines, like other polypeptide hormones, initiate their action by binding to specific receptors on the surface of target cells. One of the most well known families of cytokines are the interleukins which mediate natural immunity. For a detailed description of the structure and function of the interleukins, see Abbas et al. Cellular and Molecular Immunology, W. B. Saunders Company, Philadelphia, pp. 225–243, 1991.
IL-6 is a multifunctional cytokine having a molecular weight of 26 kDa which is produced by both lymphoid and non-lymphoid cells and regulates immune responses, acute-phase reactions and hematopoiesis. A detailed review of the structure and function of this cytokine may be found in The Cytokine Handbook, Third Edition, Thomson, A. Ed., Academic Press, San Diego, Calif., 1998, and in Barton, Clin. Immunol. Immunopathol. 85:16–20, 1997. Because many cells are capable of both producing and responding to IL-6, it is capable of being an autocrine regulator of growth and/or differentiation in many systems. Within the immune system, it has been shown to be an autocrine activator of peripheral T and NK cells, which, in part, is mediated via IL-2 (Garman et al., Proc. Natl. Acad. Sci U.S.A. 84:7629–7633). In thymic ontogeny, IL-6 may be important alongside IL-2, IL-4 and IL-7 in thymic development. IL-6 also promotes IgG secretion by activated B cells. In addition, IL-6 induces the liver to produce acute-phase proteins such as C-reactive protein and inhibits the production of albumin (Morrone et al., J. Biol. Chem. 263:12554–12558, 1988).
IL-6 is also involved in T cell activation, growth and differentiation. IL-6 induces IL-3 receptor (Tac antigen) expression in one T cell line (Noma et al., 1987) and in thymocytes, and functions as a second signal for IL-2 production by T cells (Garman et al., 1987). IL-6 promotes the growth of human T cells stimulated with PHA or mouse peripheral T cells. IL-6 also inhibits several key inflammatory responses including the synthesis of LPS-induced IL-1 and TNF- in vitro and in vivo (Aderka et al., J. Immunol. 143:3517–3523, 1989; Ulich et al., J. Immunol. 146:2316–2323, 1991). IL-6 has also been found to protect against lung damage in disease models of pulmonary inflammation (Chen et al., Infect. Immun. 61:97–102, 1993).
Neurotrophins and neurotrophic factors are proteins or peptides capable of affecting the survival, target innervation and/or function of neuronal cell populations (Barde, Neuron, 2:1525–1534, 1989). The efficacy of neurotrophins both in vivo and in vitro has been well documented. For example, ciliary neurotrophic factor (CNTF) promotes survival of chicken embryo ciliary ganglia in vitro and supports survival of cultured sympathetic, sensory and spinal motor neurons (Ip et al., J. Physiol. Paris, 85:123–130, 1991).
A major obstacle to the in vivo therapeutic use of peptides is their susceptibility to proteolytic degradation. Retro-inverso peptides are isomers of linear peptides in which the direction of the sequence is reversed (retro) and the chirality, D or L, of each amino acid is inverted (inverso). There are also partially modified retro-inverso isomers of linear peptides in which only some of the peptide bonds are reversed and the chirality of the amino acid residues in the reversed portion is inverted. The major advantage of such peptides is their enhanced activity in vivo due to improved resistance to proteolytic degradation (For review, see Chorev et al., Trends Biotech., 13:438–445, 1995). Although such retro-inverso analogs exhibit increased metabolic stability, their biological activity is often greatly compromised (Guichard et al., Proc. Natl. Acad. Sci. U.S.A., 91:9765–9769, 1994). For example, Richman et al. (J. Peptide Protein Res., 25:648–662) determined that analogs of linear and cyclic leu-enkephalin modified at the Gly3-Phe4 amide bond had activities ranging from 6%–14% of native leu-enkephalin. Chorev et al., (ibid.) showed that retro-inversion of a peptide which inhibits binding of vitronectin to its receptor resulted in one peptide which was less potent than the parent isomer by a factor of 50,000, and another peptide which was 4,000 fold more potent than the parent cyclic peptide. Guichard et al. (TIBTECH 14, 1996), teach that retro-inverso (all-D-retro) antigenic mimicry may only occur with peptides in random coil, loop or cyclic conformations. In the case of “helical” peptide, adequate functional mimicry would be expected only if the helicity was, in fact, absent under the solvent conditions used for assessing antigenic mimicry.
There is a need for IL-6-derived and neurotrophic peptides exhibiting increased metabolic stability while retaining biological activity. The present invention addresses this need.